This invention relates to ignition systems and, more particularly, to thermal clamping a coil for an ignition system.
The charging of a coil followed by a collapse of current through that coil typically produces a large voltage across the coil which can be used to fire a spark plug in an ignition system as is well known in the art. However, conditions such as the ignition system becoming too hot may occur requiring the ignition system to be protected by, for example, preventing the firing of the spark plug. Given this scenario, if the coil in the ignition system is charged, it is typically desired to slowly discharge the energy in the coil to prevent the firing of the spark plug. Also, if the voltage at the coil to a predetermined voltage so as to prevent the firing of the spark plug. Furthermore, if the coil is discharged and the ignition system is still hot, it is typically desired not to recharge the coil.
One attempt that prior art has made for protecting ignition coil drivers is disclosed in U.S. Pat. No. 4,359,652 by Jarrett et al and assigned to Motorola Inc. which describes an overvoltage detection circuit which teaches the protection against high voltage conditions occurring at a battery supply terminal. However, Jarrett does not teach any means to slowly discharge an ignition coil when the overvoltage condition occurs nor does it teach the prevention of recharging an ignition coil if the ignition system is still hot.
Another attempt that prior has made for protecting ignition coil drivers is disclosed in U.S. Pat. No. 4,358,812 by LoCascio et al and also assigned to Motorola Inc. which teaches how to switch the output current through an inductive load to a predetermined constant value when a predetermined peak magnitude of load current is reached. However, LoCascio also does not teach any means to slowly discharge the inductive load to prevent the firing of a spark plug, nor does it teach the prevention of recharging an ignition coil if the ignition system is still hot.
Hence, a need exists to provide a circuit for slowly discharging a coil in an ignition system to abate the firing of a spark plug and to prevent the recharging of the coil when the ignition system is still hot.